An arrayed waveguide grating (AWG) is a device that, via dispersion, is used to separate (demultiplex) or combine (multiplex) different optical wavelengths in, for example, wavelength-divisional multiplexing (WDM) systems. In an AWG, a flat transmission passband is desirable. That is, it is desirable that the AWG provides a uniform diffraction angle for all wavelengths inside each passband. However, the dispersion of the waveguide array, which is the mechanism that allows the AWG to separate or combine different optical wavelengths, prevents a truly uniform transitivity inside the passband.